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Bureau of Mines Information Circular/1984 




Mine Power System Analysis- Design 
Computer Programs 



By Dean H. Ambrose 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8970 

n 



Mine Power System Analysis-Design 
Computer Programs 



By Dean H. Ambrose 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 






Library of Congress Cataloging in Publication Data: 



Ambrose, Dean H 

Mine power system analysis-design computer programs. 

(Information circular ; 8970) 

Bibliography: p. 5. 

Supt. of Docs, no.: I 28.27:8970. 

1. Electricity in mining— Data processing. 2. Electric power sys- 
tems—Data processing. 3. Mining engineering— Computer programs. I. 
Title. II. Series: Information circular (United States. Bureau of 
Mines) ; 8970. 

TN295^U4^ [TN343] 622s [622'.48'0285425] 83-600373 



CONTENTS 



Page 



Abstract 1 

Introduction 2 

Computer program models 3 

Fuse coordination 3 

Prediction of ground-bed resistance 3 

Calculating intermittent-duty ampacities 3 

Load-flow fault analysis 3 

Selection of NEMA size contactor 3 

Selection of power-factor correction capacitors 4 

Relay-setting selection 4 

Reliability evaluation 4 

Snubber circuit design 4 

Prediction of transient voltage 4 

Three-phase short circuit 5 

Trip settings (Instantaneous) for trailing cables 5 

Conclusions 5 

References 5 

TABLE 

1. Computer program models 2 





UNIT OF MEASURE ABBREVIATIONS USED 


IN THIS REPORT 


A 


ampere 




kW 


kilowatt 


ft 


foot 




Mbyte 


megabyte 


hp 


horsepower 




Mvar 


megavar (million 
volt-ampere-reactive) 


kbyte 


kilobyte 




MW 


megawatt 


kV 


kilovolt 




V 


volt 


kvar 


kilovar (thou 


sand 








volt-ampere- 


reactive) 


var 


volt-ampere-reactive 



MINE POWER SYSTEM ANALYSIS-DESIGN COMPUTER PROGRAMS 

By Dean H, Ambrose 



ABSTRACT 

Conyjuter programs are presented that provide mine electrical systems 
based on computer modeling for design and safety analysis suitable for 
large or small computer systems and handheld calculators. Bureau of 
Mines research has resulted in load-flow, fault, grounding, reliability, 
short-circuit, transient, and cable ampacity computer models that enable 
mine power system engineers to analyze their system or to design a sys- 
tem. This report describes the capabilities of the program. 

^Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA, 



INTRODUCTION 



Bureau research has resulted in several 
coBoputer models that enable mine power 
system engineers to perform analyses of 
their systems. The programs are suitable 
for large and small computer systems. 
(Several programs are suitable for hand- 
held programmable calculators.) Computer 
programs have been constructed for batch 
processing (FORTRAN IV or APL) as well as 
interactive processing (FORTRAN IV, 
BASIC, or APL). 

The planning, design, and operation of 
mine power systems require several stud- 
ies to evaluate the current system per- 
formance, reliability, safety, and abil- 
ity to grow with production requirements. 
Studies of transients, reliability, 
grounding, harmonics, load-flow, short- 
circuit, and stability are most likely 
needed. The electrical engineer in 
charge of system design must decide which 
studies are needed to ensure that the 
system will operate safely, economically, 
and efficiently over the expected life of 
the system. 

The complexity of modern mine industry 
power systems has made manual performance 



of power system studies difficult and 
time consuming, if not Impossible. How- 
ever, through the use of digital com- 
puters, these studies can be made with 
relative ease. Answers to many perplex- 
ing questions regarding impact of ex- 
pansion on the system, short-circuit 
capacity, stability, load distribution., 
etc. , can be intelligently obtained. 

It is important for those concerned 
with assembling and preparing data for 
input to a power system analysis computer 
program and those interpreting and apply- 
ing results generated by such a program 
to understand the development of the pro- 
gram and of basic analytical solutions 
that apply. The following section will 
briefly discuss the purpose of each com- 
puter program (table 1). Details of pro- 
gram development and analytical solutions 
that apply program listings, and program 
run procedures, are available from the 
National Technical Information Service 
(NTIS), U.S. Department of Commerce, 5285 
Port Royal Rd., Springfield, VA 22161. A 
reference is given at the end of each 
discussion. 



TABLE 1. - Computer program models 



Program 



Language 



Processing^ 



Application 



Fuse coordination 

Ground bed design , 

Ground bed resistance , 

Intermittent duty ampacities, 

Do 

Load-flow and fault analysis, 

Do , 

Do 

NEMA size contactor , 

Power factor correction , 

Relay setting selection...... 

Reliability evaluation , 

Snubber circuit 

Transient voltages 

3-phase short circuit 

Trip settings for cables..... 



(2) 

(2) 

(2) 
BASIC , 

(2) 
FORTRAN IV. 

APL , 

BASIC 

(2) 

(2) 

(2) 
FORTRAN IV. 

(2) 

(2) 

(2) 

(2) 



HP-97, TI-58/59... 

. . .do. 

. . .do 

Interactive. 

HP-41C 

Interactive-batch, 

...do 

Interactive 

HP-97, TI-58/59... 

. . .do. 

. . .do 

Batch 

HP-97, TI-58/59... 
...do 



.do, 
•do. 



Design, 

Do. 
Analysis. 

Do. 

Do. 

Do. 

Do. 

Do. 
Design. 

Do. 

Do. 

Do. 

Do. 
Analysis, 
Design. 

Do. 



TI Texas Instruments. NEMA National Electrical Manufac- 



HP Hewlett-Packard, 
turers Association. 

^Use of brand names is for identification only and does not imply 
the Bureau of Mines. 

^Program written for handheld calculator listed under "Processing." 



endorsement by 



COMPUTER PROGRAM MODELS 



FUSE COORDINATION 

Three programs were developed to per- 
form fuse coordination design. To prop- 
erly apply fuse overload and fault pro- 
tection, it is necessary that fuses be 
applied such that they can carry the con- 
tinuous current of the system they are 
protecting. The use of the program is 
somewhat complicated because the fuse 
data is so voluminous. The input data 
required for each fuse location includes 
full-load and maximum-fault currents. 
The program performs calculations on each 
fuse location. These calculations pro- 
vide a fuse size for each location re- 
sulting in a fuse coordination scheme 
(0.2 

PREDICTION OF GROUND-BED RESISTANCE 

The program was developed to compute 
the ground-bed resistance by merely sup- 
plying a description of the ground bed. 
The use of the program is straightfor- 
ward. Regardless of the type of ground 
bed configuration, the following quanti- 
ties are entered: earth resistivity, rod 
length, rod radius, and the number of 
rods. Also, enter bed diameter if 
the ground bed configuration is a circle, 
or enter spacing between adjacent rods 
if the ground bed configuration is a 
straight line. The program calculates 
the ground bed resistance and outputs the 
results (_1_). 

CALCULATING INTERMITTENT-DUTY 
AMPACITIES 

The program was developed to determine 
trailing cable ampacities under cyclic 
loading. The program resolves technical 
issues on how to incorporate the vari- 
ables (insulation type, ambient tempera- 
ture, cable size, and cable temperature 
variation) in a meaningful yet manageable 
relationship for rating cables. The 
program required input values include 
cable cycle time, cable percentage on 

•'Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



time, cable size, and cable 30CFR18 rat- 
ing. The output prints the allowable 
current a given cable can safely carry 
under cyclic loads (2). 

LOAD-FLOW FAULT ANALYSIS 

Two programs were developed to perform 
analysis on coal mine electrical power 
systems under either normal operating 
conditions or fault situations. Both 
programs provide options for choosing a 
solution technique. The programs have 
the capability for analyzing of systems 
having 100 buses and 300 elements, al- 
though these numbers could be changed, if 
desired. 

The LOAD-FLOW program required input 
data include specification of mine power 
system topology, cable sizes and lengths, 
load horsepower, transformer ratings, and 
per unit impedances. The output includes 
convergence data, magnitude and angle of 
the voltage at each bus, power flows and 
line current flows between buses, and 
power and current flows to ground at each 
bus. 

The FAULT program required input data 
includes mine power topology, cable sizes 
and lengths, transformer ratings and per 
unit impedances, and load horsepower. 
The output contains the fault current for 
the specified fault type for a fault at 
each system bus. For balanced faults, 
the voltages at all system buses and the 
line current flows between all buses are 
printed for a fault at each bus selected. 
For line-to-line faults, this information 
is printed for all buses connected to the 
faulted bus (}rl^ ' 

SELECTION OF NEMA SIZE CONTACTOR 

The program was developed to compute 
the correct National Electrical Manufac- 
turers Association (NEMA) size contactor 
for a particular size motor. The program 
is quite accurate for motor sizes between 
2 and 600 hp. Outside this range, a 
table reference procedure is used. The 
program is easy to use: One needs only 



to input the motor horsepower rating, 
assuming the voltage rating is 460 to 575 
V. The program prints out the minimum 
size starter required O). 

SELECTION OF POWER-FACTOR 
CORRECTION CAPACITORS 

The program was developed to compute 
the kilovars of capacitors required to 
increase the power factor of a known load 
to a higher value. This program uses an 
analytical approach equivalent to the 
table of multipliers. In order to use 
this program, input the original power 
factor, desired power factor, and real 
power load in watts, kilowatts, or mega- 
watts. The program prints out the capac- 
itor rating that is required in vars, 
kilovars, or megavars, depending upon the 
units for the real power (J_)» 

RELAY-SETTING SELECTION 

In order to be sure that the protecting 
relay causes its breaker to operate with- 
out the protected relay tripping its 
breaker, it is necessary that the relay 
times differ by a selective time margin. 

This program was developed to compute 
time-dial settings of relays that nor- 
mally use time-current plots to determine 
the settings. When relays of the same 
type are used, the time margin between 
breaker operating time and the factor of 
safety always decreases with increasing 
current. The selective time margin 
between two relays always exist at the 
maximum available fault current level to 
assure proper coordination for worst case 
conditions. This program allows the 
selection of time-dial settings that will 
achieve the desired selective time mar- 
gins without drawing time-current co- 
ordination graphs. The program requires 
the values of 14 constants for each type 
of relay. These constants are available 
on any relay specification sheet. Be- 
cause the program provides for storage of 
those values, they do not have to be 
keyed in each time they are used. The 
output prints the results of delay time 
when overcurrent is given, delay time 
when dial setting is given, or dial set- 
ting when delay time is given (1). 



RELIABILITY EVALUATION 

The program was developed to coii5)ute 
the failure rate and forced unavailabil- 
ity for any system configuration for a 
maximum of two redundancies. The program 
is fairly general so long as the elements 
of the system are symmetrical (i.e., 
cable, transformers, but not rectifiers). 
The failure rate data of molded-case cir- 
cuit breakers, metal-clad drawout break- 
ers, serial cables (15 kV at 1,000 ft), 
cable terminations, transformers, pro- 
tective relays, disconnect switches, 
fuses, and insulated buses are already 
stored in the program. All one has to do 
is to number the various nodes of the 
system and enter the pertinent component 
data (node connections, conqjonent type, 
etc, ) into the program. The program 
prints out the equivalent failure rate, 
downtime per failure, and the total down- 
time at each load point (_5), 

SNUBBER CIRCUIT DESIGN 

The program was developed to design a 
low pass filter for transient sup- 
pression. The program provides an alter- 
native to the nomogram technique normally 
used in the design of these filters. The 
input data required include the follow- 
ing: transformer rating, frequency, sys- 
tem series inductance, and allowable peak 
transient voltage. The program prints 
out four values to get the snubber 
design: capacitance, resistance, maximum 
power dissipated in the resistance, and 
working voltage of the capacitor il)» 

PREDICTION OF TRANSIENT VOLTAGE 

The program determines abnormal tran- 
sient voltages on power systems. The pro- 
gram has three separate options (solu- 
tions): compute the value of a standard 
curve for a specific value of normalized 
time, compute the value on a specific 
standard curve (maximum voltage and cur- 
rent), and compute the damping factor for 
a given peak value of current or voltage. 
The program is dependent on reducing the 
transient problem to a configuration that 
can be represented by either a series or 
parallel RLC circuit (1), 



THREE-PHASE SHORT CIRCUIT 

The program was developed to compute 
the magnitude and phase angle of the 
fault current. When a three-phase short 
circuit occurs on a radial system, the 
fault current can be found by adding up 
all the contributions to the series im- 
pedance. The input data required include 
the following: system line-to-line volt- 
age, the utility short circuit level, the 
transformer data [rating, percent re- 
actance, percent resistance, and con- 
ductor data (operating voltage, react- 
ance, and resistance)] (1). 



TRIP SETTINGS (INSTANTANEOUS) FOR 
TRAILING CABLES 

The program was developed to determine 
the instantaneous settings for circuit 
breakers based on the size of trailing 
cables. The program works for AWG sizes 
from No. 14 to 3/0, except No. 10 and 
2/0. Settings for cable sizes 4/0 and 
larger are all equal to 2,500 A; the pro- 
gram handles this automatically. In 
order to use the program, one needs only 
to enter the wire size. Since there is 
no error checking, it is important that 
the wire sizes be properly entered (1). 



CONCLUSIONS 



The relatively low cost of minicom- 
puters and microcomputers has contributed 
to their abundant use in the mine in- 
dustry. The programs described in this 
report were written for use on a small 
coin)uter having 256 kbytes of random 
access memory (RAM) and approximately 
1/2 Mbyte of working disk storage. 
Also, several programs were written for 
use with a handheld calculator having 
magnetic-strip storage capability. De- 
spite their limitations, the programs are 
capable of analyzing or designing the 



electrical power system for a mining 
operation in a reasonable amount of time. 

The digital computer has already proven 
its value in other industries. The com- 
putational tasks involved in load-flow 
studies, short-circuit calculations, and 
transient analyses have been greatly sim- 
plified by the use of computers. Now, 
analyses and design of the electrical 
power system of a mine can also be 
effectively performed by computer. 



REFERENCES 



1. Stanek, E. K. , and M. Cabert. Mine 
Electrical Power Systems. Transients 
Protection, Reliability Investigation, 
and Safety Testing of Mine Electrical 
Power Systems (contract G0144137, WV 
Univ.). Volume IV: Use of Programmable 
Calculators in Mine Power System Designed 
Analysis. BuMines OFR 6(4)-81, 1979, 117 
pp.; NTIS PB 81-166795. 



2. U.S. Bureau of Mines. 
Coal Mine Power Systems. 
Bureau of Mines Technology 
inar, Pittsburgh, PA, Sept. 
8893, 1982, 88 pp. 



Underground 

Proceedings : 

Transfer Sem- 

16, 1982. IC 



3. Trutt, F. C, L. A. Morley, 
and R. A. Rivell. Interactive 



Mine-Power-System Analysis. Volume 1 
(contract J0199060, PA State Univ.). 
BuMines OFR 162(1)-81, 1981, 207 pp.; 
NTIS PB 82-138298. 

4. Trutt, F. C. Interactive Mine- 
Power-System Analysis. User's Manual. 
Volume 2 (contract J0199060, PA State 
Univ.). BuMines OFR 162(2)-81, 1980, 237 
pp.; NTIS PB 82-138306. 

5. Stanek, E. K. Enhancement of Mine 
Power System Safety and Reliability 
(contract G0188097, WV Univ.). Bu- 
Mines OFR 116-80, 1979, 203 pp.; NTIS 
PB 81-125361. 



INT.-BU.OF MINES, PGH., PA. 27 36 2 



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